Function is_envelope_row 

pub fn is_envelope_row(message: &Message) -> bool

Whether a frame is a bracket-led row: the shape every envelope this workspace injects opens with - a delivery, a peer wrapper, a failure.

The prefix alone, on purpose. The parser that tells a real envelope from the reader’s own [note] row lives above this crate, and the two readers want opposite answers about it: the cut must never start on a frame the fold opens nothing at, while the debt must not carry a previous turn’s counters into the newest. So the cut leaves every bracket-led row alone and the debt counts every one as an opening - the debt’s error is then a figure short of counters rather than one holding another turn’s.